The conservation land was only part of the mine as a whole, a further 104 hectares of land sits in the Westport Water Conservation Reserve where decision making rights are held by the Buller District Council.

Ms Sage and Ms Woods said they decided against the application, as any economic benefits would not outweigh the likely damages.

"The Te Kuha area is one of the last two areas of intact, elevated Brunner coal measure ecosystems," Ms Sage said.

"It is an undisturbed area which is precious and unique and supports complex and diverse habitats for threatened plants and wildlife including great spotted kiwi, land snails and lizards.

"By altering the existing topography and ridgeline the proposed mine would also affect the scenic values and landscapes of the Lower Buller Gorge."

Ms Woods said the proposal had been carefully considered under Crown Minerals Act.

"We felt that in this instance that the irreversible and permanent costs to the natural environment were too high and outweighed the economic benefits from the proposal," she said.

Environmental advocacy group 350 Aotearoa called the decision a win.

"[It's] a win for people-power, and is completely necessary for the Government to live up to its promise to New Zealanders to take action on climate change."